The present invention relates generally to power transmission arrangements for tractors.
More particularly, the invention relates to the power transmission arrangement in a four-wheel drive tractor wherein power is transmitted from the tractor gearbox or from a separate power take-off gearbox to the differential gear of the front axle by means of a cardan shaft or similar power transmission member and wherein the tractor is provided at its front end with a power take-off shaft for transmitting power from the engine crankshaft to external equipment.
The power transmission arrangement of a conventional four-wheel drive tractor is generally adapted to transmit power from the engine, situated at the front end of the tractor, through a drive clutch and a cardan shaft or clutch shaft to the gearbox situated at the rear end of the tractor. The gearbox may be situated in the same housing along with the drive gear of the rear wheels. The gearbox may be provided with a clutch and with a power take-off for the four-wheel drive, or with a separate take-off box, from which power is transmitted through a cardan shaft to the drive gear of the front axle. The front axle is usually a pivoted or swinging axle which is mounted on the tractor frame and normally comprises a rigid, hollow axle housing in which the drive gear is installed.
A general drawback of a swinging axle is that not much room is available within which the axle can tilt since, upon tilting, the axle soon comes into contact with the tractor frame. Moreover, in the particular case of four-wheel drive tractors whose front wheels are usually relatively large, tilting of the front axle may cause the wheels to come into contact with the engine housing. When the housing is tilted, the wheels are of course also tilted which reduces the grip and tractive capacity of the wheels.
In order to improve the traction and grip of the front wheels, a differential lock or so-called "limited slip" type friction lock may be used in connection with the differential gear of the front axle. Generally, a hydraulic or electrically operated positive lock cannot be used on the front axle because the hoses and wires will tend to follow the profile of the terrain over which the tractor moves so that in the case of rough and uneven terrain, the hoses and wires can be broken. Moreover, hydraulic locks frequently have small oil leaks and any oil leakage would eventually find its way into the axle housing and possibly result in damage to the axle seals.
In tractors provided with power take-off means at the front end of the tractor, the arrangement is such that the power is obtained from the engine crankshaft by means of a cardan shaft coupled to a separate reduction gear of the power take-off transmission which is provided with a power take-off shaft. The reduction gear of the power take-off transmission arrangement is normally situated in a separate housing which also contains the clutch for the power take-off transmission arrangement. The clutch normally comprises a dry friction clutch although so-called oil bath clutches may also be used. One of the drawbacks of a separate power take-off gearbox is that it requires an oil location which is separate from the other parts of the tractor which again results in additional points on the tractor which require servicing and which may constitute a point of oil leakage.